added HTML.encode (in Scala), similar to HTML.output in ML;
added HTML.encode (in Scala), similar to HTML.output in ML;

MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle